Ben Finney wrote: > How can I ensure incidental names don't end up in the class definition, > with code that works on both Python 2 and Python 3? > > With the following class definition, the incidental names `foo` and > `bar`, only needed for the list comprehension, remain in the `Parrot` > namespace:: > > __metaclass__ = object > > class Parrot: > """ A parrot with beautiful plumage. """ > > plumage = [ > (foo, bar) for (foo, bar) in feathers.items() > if bar == "beautiful"] > > assert hasattr(Parrot, 'plumage') # ← okay, has the wanted name > assert not hasattr(Parrot, 'foo') # ← FAILS, has an unwanted name > assert not hasattr(Parrot, 'bar') # ← FAILS, has an unwanted name > > So I can remove those names after using them:: > > __metaclass__ = object > > class Parrot: > """ A parrot with beautiful plumage. """ > > plumage = [ > (foo, bar) for (foo, bar) in feathers.items() > if bar == "beautiful"] > del foo, bar > > assert hasattr(Parrot, 'plumage') # ← okay, has the wanted name > assert not hasattr(Parrot, 'foo') # ← okay, no unwanted name > assert not hasattr(Parrot, 'bar') # ← okay, no unwanted name > > But that fails on Python 3, since the names *don't* persist from the > list comprehension: > > __metaclass__ = object > > class Parrot: > """ A parrot with beautiful plumage. """ > > plumage = [ > (foo, bar) for (foo, bar) in feathers.items() > if bar == "beautiful"] > del foo, bar # ← FAILS, “NameError: name 'foo' is not defined” > > How can I write the class definition with the list comprehension and > *not* keep the incidental names — in code that will run correctly on > both Python 2 and Python 3?
If you absolutely must: make sure that the names exist by setting them explicitly: class Parrot: per = None a = [... for per in ...] del per But I would probably use a generator expression. These don't leak names: Python 2.7.6 (default, Jun 22 2015, 17:58:13) [GCC 4.8.2] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> class Parrot: ... a = [per for per in "abc"] ... b = list(trans for trans in "def") ... >>> Parrot.per 'c' >>> Parrot.trans Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: class Parrot has no attribute 'trans' -- https://mail.python.org/mailman/listinfo/python-list
